Conventionally known is an autonomous underwater vehicle (hereinafter may be referred to as an “AUV”) which does not require electric power supply from a mother ship and sails in water by a built-in power source for seabed work, seabed investigation, and the like. After the AUV is carried to a target marine area by the mother ship, the AUV is put into the sea from the mother ship and performs predetermined work under the sea surface. After the predetermined work, the AUV floats to the sea surface and is lifted to the mother ship.
Known as a typical method of lifting the AUV to the mother ship is a pop-up buoy method of lifting the AUV to the mother ship by: collecting a buoy, connected to a main body portion of the AUV by a rope, to the mother ship; and then winding the rope. For example, PTL 1 discloses that the AUV is lifted in such a manner that: when the AUV floats from under the sea surface, the AUV discharges a buoy with a rope; the buoy is collected by throwing a sand weight or the like from the mother ship and hooking the sand weight on the buoy; and then, the rope attached to the buoy is wound.